There are many different methods of generating a clock that is an integral multiple of a fundamental frequency. One well known method is the use of a PLL and a frequency-phase detector with a feedback loop. Such devices are referenced in U.S. Pat. No. 5,809,291 and the book "Phase Locked Loops" by Floyd Gardner. One problem with phase locked loops is that the phase noise of the generated output signal is derived from the phase noise of the VCO which generates it, rather than from the low phase noise of the fundamental frequency oscillator which feeds it. This results in an interesting phenomenon whereby the phase noise of the output signal actually decreases very close to the fundamental frequency, where the lower noise and high loop gain of the error amplifier is able to produce an error signal which tracks the phase error of the VCO and feed this signal back into the noisy VCO, thereby making this part of the spectrum quieter. The VCO typically has a higher phase noise level than a crystal oscillator because the crystal oscillator has a much higher Q and is only able to operate in a very narrow range of frequencies, while the VCO has an intrinsically lower Q and can be modulated by small levels of electrical noise appearing on the control voltage input.
An alternate method of deriving a higher frequency signal from an input signal is the use of DLL (delay locked loop) oscillator, which comprises a chain of delay elements in a loop, each delay element controlled by a control voltage. Such systems are described in U.S. Pat. Nos. 5,818,270 and 5,920,211. In this type of system, the DLL is providing the intrinsic phase noise, and the system clock is merely a reference for the use of the comparator.
Another alternate method of deriving a higher frequency signal from an input signal involves the use of an exclusive OR gate with a delay line, as is described in U.S. Pat. No. 5,563,538. In this circuit, the objective is not the generation of a harmonic clock, but the extraction of a 2.times. clock from a clock-encoded optical signal of varying amplitude, typically from a photodiode of a communication system. In this reference, a delay line is combined with an exclusive OR gate to produce a 50% duty cycle signal at 2.times. the input frequency, and a SAW bandpass filter of high Q is provided afterwards to provide a sinusoidal output waveform at twice the output frequency.